In the related art, an electromagnetic induction type position detector is known that includes a plate-shaped graduation substrate having graduation coils arranged with a predetermined pitch along a predetermined direction and a detector having a transmission coil and a reception coil that are provided to face the graduation coil. The electromagnetic induction type position detector is used, for example, for calipers, indicators, linear scales, micrometers and the like. By driving (exciting) the transmission coil, the electromagnetic induction type position detector detects, via the graduation coil, a sine wave signal (signal) having the same period as the pitch of the graduation coil with the reception coil. The electromagnetic induction type position detector calculates, from the detected signal, the position of the detector in a predetermined direction with respect to the scale substrate.
For example, in Patent Document 1, an electromagnetic induction type encoder (electromagnetic induction type position detector) includes a scale (graduation coil) having a scale coil (graduation coil) and a head (detector) having a transmission coil and a reception coil. In such an electromagnetic induction type encoder, a scale coil, a transmission coil, and a reception coil are constituted by using a multilayer substrate formed by stacking a plurality of substrates.
Here, an incremental method (INC method) and an absolute method (ABS method) are known as methods for calculating the position of the head in a predetermined direction with respect to the scale by using the electromagnetic induction type encoder.
The INC method is a method in which a relative position is calculated by continuously detecting incremental graduation coils (INC graduation coils) of a scale that are arranged at a constant pitch, and counting up or counting down the number of coils in the INC graduation coils that pass. However, the INC method has a problem in that the absolute position of the head in a predetermined direction with respect to the scale cannot he acquired.
In contrast, the ABS method is a method in which an absolute position is detected by detecting, with the head at an appropriate timing, absolute graduation coils (ABS graduation coils) that are randomly arranged on the scale, and analyzing the detected signals. In the ABS method, it is possible to acquire the absolute position of the head in a predetermined direction with respect to the scale. However, the ABS method has a problem in that, although the absolute position can be acquired, it is only possible to acquire position information with a lower resolution than that of the INC method.
To address such a problem, Patent Document 2, for example, proposes an encoder (electromagnetic induction type position detector) that uses the calculation methods of both the INC method and the ABS method. The encoder uses a double-track type scale provided with an incremental track (INC track) that includes an INC pattern (INC graduation coil) and an absolute track (ABS track) that includes an ABS pattern (ABS graduation coil). Then, the INC pattern and the ABS pattern are detected by a detector head (detector), and the position information is calculated based on each pattern.
This encoder compares the relative position calculated from the INC pattern with the absolute position calculated from the ABS pattern. Next, the encoder combines the relative position and the absolute position to calculate the position of the head in the predetermined direction with respect to the scale, verify the detection error by comparing the relative position and the absolute position, and correct the relative position by using the absolute position.
In a case where it is desired to calculate a higher resolution position of the head in the predetermined direction with respect to the scale by using the INC method and the ABS method, it is conceivable that the electromagnetic induction type encoder could configure a shorter period of the signal to be detected. In particular, in the electromagnetic induction type encoder, by configuring the pitch of the scale coil, the transmission coil, and the reception coil to be small, for example, the period of the signal can be configured to be short.